It has a niche audience. I'd be surprised if it sold 1,000 copies at any price. But if I were interested in the subject, $50 is not a lot of money if I thought it would get me on the ground running. I paid for Discover Meteor, for example.
Meh, how about the variance in purchasing power and cost of living among all the places that use just one currency? You'll find differences even greater than $8.
Prices are arbitrary. Locking USD = EUR makes you superficially feel like you're getting ripped off but it's still arbitrary. You're also paying the same as someone making $100k/year more or less than you.